Disc brakes are known which exhibit a shoe with a carrier plate and a friction lining. During braking, the shoe presses against the brake disc. By this means, over time the friction lining is subject to wear.
For monitoring of the friction lining width, especially for determining whether a permissible wear limit of the friction lining has not been reached, it is known to place electrical contact wires—cables, for example—in the friction lining. The contact wire is mounted roughly in a radial outer area of the brake shoe on the carrier plate. This area often is free of the friction lining. Thus, for this, the friction lining often exhibits an opening. When the wear limit is reached, thus when the friction lining has been worn away accordingly, the contact cable comes in contact with the brake disc during braking, and is also abraded. By this means an electrical impulse is issued to a signal detector, and a warning notice is issued that the permissible residual shoe thickness has been reached. In this case the shoe must be replaced.
Various modes are known by which the electrical contact cable is situated in a housing and of how the housing itself is designed, for example to be attached to the carrier plate. The mounting should be as durably stable as possible, so that the positioning of the electrical cable is reliable and constant. Any alteration of the position is undesirable, because it will affect the indication. For example, it can result in the warning notice being given too early or too late. As affected by operation, the display device is subject to extremes of temperature. Therefore the materials used for it (such as plastic) must be of high quality.
Display devices are known in which the carrier plate is provided in the radially outer area with a smooth or a stepped aperture in the form of a slit. In this case, the sensor housing is compressed in radially from without into the slit. However, this type of positioning has a drawback in that operational access is not possible, because systems that hold down the shoe, such as compressing springs mounted on the carrier plate, are in the way.
From EP 692 652 A1, a wear indicator—here insulated bodies—is known which are axially compressed via a projection into a hole or a recess of the carrier plate. Axial securing is done by partially form-locking expansion, and multiple procedural steps are required for positioning. Axial pre-tensioning accomplishes the rotational securing. Only a single electrical cable is embedded (cast) in the insulated bodies, which runs radially outward axially in front of the carrier plate. In terms of manufacturing technique, this type of embedding is very expensive and also inflexible. The client's requirements for differing cable lengths must be allowed for during the casting.
A further drawback is the single-cable design. Due to that, no state-of-the-art diagnosis can be made, such as for example display, storage and query, in the vehicle as regards the functional capability of the wear display and regarding the degree of wear. Here a signal is only triggered when the cable touches the brake shoe during braking and is abraded.
DE 38 20 977 A1 exhibits multiple attachment options and sensor designs. According to a first embodiment form, a sensor of multi-component design with guidance of an open contact loop is inserted from the coating-free rear side of the carrier plate and attached there. According to another embodiment form, it is inserted radially from without. According to the first embodiment form, a part of the mounting and the cable itself extends into an area in which either clamping devices or brake caliper legs press against the carrier plate. This limits the application in state-of-the-art disc brakes for commercial vehicles, and also it is not possible to retrofit the sensor with installed brake shoes, because there is no access.
In the embodiment form with radial installation, the carrier plate must be equipped with multiple steps to make a mounting possible at all, and so that the U-shaped contact loop can be reset far enough back in the area of the carrier plate that the cable area pointing to the brake disc can display a justifiable friction lining width. But stepped carrier plates require expensive manufacturing. The steppings are also necessary if both of the legs of the U-shaped contact loop lie directly behind each other and thus are aligned transverse or perpendicular to the brake disc plane. For positioning, this kind of cable progression requires a corresponding space for installation, which has a disadvantageous effect on cost-effective utilization of the friction lining.
As shown by the preamble of claim 1, the invention is based on the teaching of DE 38 20 977 A1.
The task of it is to improve the display device according to DE 38 20 977 A1 so that the required installation space is reduced.
According to the invention, the set problem is solved in that the electrically conducting device is elongated and a section that adjoins the section that in an installed state lies closest to the brake disc forms an angle α with the plane of the brake disc which lies between 0° and 90°, preferably between 2° and 20°, and additionally preferred between 8° and 16°.
In other words, for example in the case of a U-shaped contact loop, what is avoided is that the two legs, viewed in the axial direction of the brake, lie directly behind each other, taking up a large amount of axial structural room. Rather, for example, by the above-mentioned adjoining section forming an angle α with the plane of the brake disc between 0° and 90°, the axial structural space is reduced, because the two legs of a U-shaped contact loop are placed offset to each other. By this means, as viewed in the axial direction, they can be situated considerably closer to each other, thus causing the axial structural space to be reduced.
It should be indicated expressly here that the angle α is “between” 0° and 90°, and therefore the two limit values of 0° and 90° are precluded. At 90°, in the case of the U-shaped contact loop, the two legs of the contact loop would lie axially behind each other, as is the case according to DE 38 20 977 A1. At 0°, the two legs, in the case of a U-shaped contact loop, would be next to each other in a plane lying parallel to the plane of the brake disc, and therefore after reaching the wear limit, they would be jointly abraded and worn. In such an instance, it can happen that the two cable arms simultaneously would lose holding contact and either would be destroyed by the brake disc or not even come in contact at all with the brake disc. Thus incorrect information would be issued and it would be impossible to diagnose for functionality.
This cannot occur with the invention-specific configuration. In the case of the U-shaped contact loop, the two legs are not merely at a lateral interval, they are also offset axially to each other. Therefore, one of the two legs is less worn than the other, and therefore it can still be held securely. This thus ensures permanent positioning, through which signal reliability is considerably improved. Simultaneous contact wear and wear-through are avoided, resulting in a better holding of the “residual cable” at the display point.
A further advantage as compared to cable legs lying behind each other in the case of the U-shaped contact loop is that despite the required cable cross sections, due to the offset, and while maintaining secure contact as viewed in the axial direction, the two U legs can lie relatively close to each other, through which it is possible to display an effective and economical residual shoe thickness. Only thus can a residual shoe thickness be signaled which corresponds to state-of-the-art requirements for maximum shoe implementation. This is especially so for commercial vehicles, where, for large-area brake shoes, with shoe thickness to match, a residual shoe thickness of about 2 mm is required.
According to the invention, preferably provision is made that the electrically conducting device carries current when in operation and in terms of potential when the brake is not activated, is uncoupled from the brake disc, and the current correspondingly is interrupted upon reaching a second wear limit behind the first wear limit.
Provision can also be made to direct a test current through the electrically conducting device, to check its functional capability.
Especially in the case of a U-shaped contact loop, in this case the cable leg that lies closer to the brake disc comes in contact with the brake disc first. If it is abraded, then it is drawn to the electric potential of the brake disc (for example body contact). Thereupon for example an (interrupted) warning signal is generated. The cable leg that lies closer to the brake disc issues a signal when the first wear limit is reached. This first wear limit can be so set that when it is reached, the permissible residual shoe thickness is approximately reached. If the cable leg lying closer to the brake disc is destroyed due to further abrasion, then the current through the cable loop is interrupted. This interruption of current can be used to generate an additional warning signal, for example one which persists longer. The second wear limit thus reached can be equal to the maximum permitted wear (minimum friction lining thickness).
Further preferred provision is made according to the invention that the section of the electrically conducting device that lies closest to the brake disc, upon reaching the first wear limit during braking, adjoins the brake disc, so that in the interval between the first wear limit and the second wear limit it is destroyed.
In particular, this configuration permits the functions explained individually above.
According to another preferred embodiment form of the invention, an opening is provided in a carrier plate of the brake shoe, which aligns with an opening in the friction lining, with the two openings at least partially admitting a holder for the electrically conducting device.
By this means it is possible for the invention-specific display device to be designed with particular simplicity.
Further preferred according to the invention is the opening in the friction lining that is open radially outwards. In other words, the friction lining exhibits a recess.
According to the invention, the opening (recess) in the friction lining in a further preferred fashion has the shape of a channel, especially a channel that tapers down toward the carrier plate.
This configuration facilitates deliberate axial sliding in of the holder from the side of the friction lining, for example up to the stop on the carrier plate. This facilitates and improves the positioning.
If the channel is configured to have a taper, this improves the attachment of the holder and positioning of the electrically conducting device, because a relative form-locking is generated.
Further preferred according to the invention is that the opening in the carrier plate supports the holder. By this means, a particularly simple design is implemented.
Additionally preferred is a self-acting locking of the holder the placement of the holder in response to placement of the holder into the carrier plate opening.
Such an axial securing of the holder can be configured directly or indirectly. With an indirect configuration, between a shoulder connector and a back side of the holder, a clamping plate for example is placed, which exhibits a plurality of spring-loaded tongues distributed around the circumference, which alternately point in opposite directions and are bent inward and outward. By this means the holder can be inserted so as to slide into the opening, while in contrast the spring-loaded tongues provide locking in the direction of removal. Manufacture of such a clamping sheet from a rolled band of stainless spring steel or the like is relatively simple.
Provision can also be made as part of direct mounting to embed spring-loaded tongues in the actual holder.
In both cases, a force- or form-locked mounting is present.
In contrast, a material-locked mounting can also be provided, for example by adhesive bonding, preferably with a high-temperature-resistant adhesive being used.
In additionally preferred fashion, the invention-specific device is characterized by a device based on form-locking to secure the holder against a torsion relative to the opening(s) in the carrier plate and/or the friction lining. For this purpose, for example provision can be made that the holder with at least one of its outer surfaces in its mounted state adjoins at least one inner wall of the recess in the friction lining. This solution in turn is especially simple.
According to another preferred embodiment form of the invention, provision is made that the holder exhibits a first holder channel for the section of the electrically conducting device which in the installed state is closest to the brake disc, a second holder channel for another section of the electrically conducting device placed farther from the brake disc, and a connecting channel for a connecting section of the electrically conducting device.
In other words, according to this configuration, the display device is configured for example in the form of the already above-mentioned U-shaped contact loop.
Especially with this configuration, the holder is designed to have an L-shaped cross section of the base form. One of its legs serves as an axial shoulder for mounting in the opening of the carrier plate. The area of the holder lying closer to the brake disc admits the contact loop.
Further preferred according to the invention, the connection channel, the first holder channel and/or the second holder channel have the form of an open channel.
As already mentioned above, often the length of the electrical device depends on how it is used. With closed channels in which the electrical device is embedded, especially compressed or poured, little flexibility exists in this respect, which is why expensive warehousing is required.
If the connection channel and/or the second holder channel are configured in the form of an open channel, then the (for example) thermally insulated cable of a length specified by the client can be inserted into the holder channel, to complete the device.
According to another embodiment form of the invention, provision is made that the depth of the channel is equal to, or greater than, the thickness of the section of the electrically conducting device held thereby.
With this configuration, the danger of damage is small despite the channel being open, because the electrical device is “sunken” into the channel, and by this means is protected.
In other respects the configuration can also be chosen so that the open side of the channel adjoins a surface of the recess, through which the originally open channel is closed.
Especially implementing the second holder channel in the form of an open channel, preferably open laterally, facilitates mounting. By means of suitable cross sectional contours that taper or are offset to each other—depending on the cable dimension—the cable can be accommodated and clamped therein in a positionally and functionally safe manner. Thus the insertion of the cable can be improved while maintaining functional safety.
If the first holder channel also has the form of an open channel, after insertion into the holder channels, the two sections of the electrically conducting device that project out of the channels can be twisted relative to each other for improved mounting.
In its installed state, the electrically conducting device is at a small interval from the brake disc as a carrier plate of the brake shoe.
In other words, the electrically conducting device does not lie within the carrier plate, which has substantial advantages as regards the overall structure. What is particularly achieved is independence of whether there is present a system for holding the brake down (in whatever configuration), and whether the electrical device is placed above it or below it. In addition, the electrical device also does not run in the contact area of a possible clamping system or a leg of the brake caliper. Thus, it is also possible to either retrofit the wear display device in a brake with an already installed brake shoe, or replace a possibly defective display device.
The invention is usable independent of whether the electrical device is sheathed or not. Preferably it is surrounded by a heat-resistant shielding. Also, preferably the holder is made of a heat-resistant material Especially a plastic such as a polyimide comes into consideration as a material. A duroplast is also conceivable, however.
Although the invention concept has been described above using cables laid at intervals to each other and offset relative to each other, with an axial attachment, it is also applicable in the case of radial attaching parts or fastenings corresponding to the above-described prior art. With these also the invention-specific advantages are attainable. For also in such an instance the functional safety is increased and the versatility is improved.
Express reference is made to the fact that the invention especially is applicable to commercial vehicles.
Along with the above-mentioned wear display device, the invention also provides a disc brake with such a wear display device.
Lastly, the invention also provides a brake shoe for such a disc brake.